1. Field of the Invention
This invention relates generally to a pollution control catalyst for treating the exhaust gases. More particularly, this invention relates to an improved catalyst which has a mutual protective distributed active layers each including noble-metal atoms stably structured and protected therein in the distributed-active-combination structure. The improved catalyst is stable, has high conversion efficiency, high resistance to catalyst poisons, and high temperature sustainability.
2. Description of the Prior Art
Continuous improvement of current technology in catalytic converters for purifying exhaust gas is limited by the technical difficulty that a high catalytic reactivity, which produces high conversion efficiency, often leads to lower operational stability due to low resistance to catalyst poisons and low sustainability when operated under high temperature. The demand for providing a solution is becoming more urgent due to the increasing number of automobiles thus causing the release of larger amounts of exhaust gases containing nitrous oxides (NOx), hydrocarbon (HC) and carbon monoxide (CO) and other types of pollutants into the atmosphere. The air polluted by the released exhaust gases poses serious threat to environment and human health. Therefore, the development of a highly effective catalytic converter which is capable of stable long term operation and can be manufactured at reasonable low cost has been a target for many research and development (R&D) projects.
Various types of catalyst are disclosed for treating the exhaust gases released from the internal combustion engines in order to remove the pollutants such as the hydrocarbon, carbon mono-oxide, and nitrous oxides. Saito et al. disclose in an U.S. Pat. No. 4,749,671, entitled `Exhaust Gas Cleaning Catalyst and Process for Production Thereof` (Issued on Jun. 7, 1988), an exhaust gas cleaning catalyst composed of a refractory three dimensional structure which supports a catalytically active substance. The surface or part of the catalyst which is in contact with the exhaust gas is formed with numerous irregularly arranged protrusions composed of a refractory inorganic powder with different particle sizes. The catalyst according to Saito et al. is able to increase the efficiency by increasing the contact surface between the fine particles and the exhaust gas. It is also more effective in capturing the fine carbonaceous particles from the internal combustion engines. The invention disclosed by Saito et al. where the fine particles of the active catalyst are deposited on the surface for increasing the contact of the catalyst with the exhaust gas does not provide a solution to the problems that a high reactivity catalytic material is generally more vulnerable to the instability caused by either low resistance to catalytic poisons, low temperature sustainability, or low resistance to mechanical vibrations.
In another U.S. Pat. No. 4,492,769, entitled `Pollution Control Catalyst for Internal Combustion Engine Exhaust System/Catalytic Converter and Process for Its Preparation` (Issued on Jan. 8, 1985), Blanchard et al. disclose a catalyst for a catalytic converter which is prepared by the steps of (a) coating and impregnating a support with a catalyst or palladium and at least one base metal element, (b) activating the catalyst at a temperature ranging from 120.degree. to 800.degree. C., (c) coating or impregnating the activated layer with another layer which has at least one platinum group of precious metal other than palladium and at least one other platinum group precious metal, and (d) activating the layer formed in step (c). The technique disclosed by Blanchard et al. may provide improvement in manufacturing process of palladium containing catalyst. It can be used to reduce the time and cost for producing the catalyst, but the technique does not provide a solution to overcome the basic technical difficulty that the active catalytic layer is vulnerable to the attack of the catalytic poisons and often becomes unreliable after being continuously operated under high temperature and vibration environment.
Hench et al. disclose in another U.S. Pat. No. 4,868,148, entitled `Layered Automotive Catalytic Composite` (Issued Sep. 19, 1989), a catalytic composite for treating an exhaust gas from an internal combustion engine. The catalytic composite includes a first support which is a refractory inorganic oxide having dispersed thereon at least one noble metal component selected from the group consisting of platinum, palladium, rhodium, ruthenium and iridium and having dispersed immediately thereon an overlayer comprising at least an oxygen storage component and a second support which is a refractory inorganic oxide. The catalytic layer is separated from the oxygen storage component to minimize the formation of H.sub.2 S over a catalytic composite. The unpleasant odor generated from H.sub.2 S is greatly reduced by the use of this catalyst. The layered structure disclosed in this patent however still does not provide a solution to overcome the technical difficulty caused by the vulnerability of the active catalytic compounds when subject to continuous operation environment as described above.
In another U.S. Pat. No. 5,164,350, entitled `Catalyst Composition for Purification of Exhaust Gas, Catalyst for Purification of Exhaust Gas and Process for Producing Said Catalyst` (Issued on Nov. 17, 1992), Abe et al. disclose a catalyst composition including a high silica zeolite having a Si/Al ratio of 40 or more, subject to ion exchange with at least one metal selected from Pt, Pd, Rh, Ir, and Ru, and a heat resistant oxide containing at least one metal selected from Pt, Pd, Rh, Ir, and Ru. The catalyst composition also includes a monolith carrier to support the catalyst thereon. The catalyst by Abe et al. is to provide a three way catalyst having sufficient heat resistance and processing high activity for purification. Meanwhile, according to Abe et al., the amount of expensive Rh component support on the carrier is reduced. The catalyst as disclosed by Abe et. al. provides apartial solution to the problem encountered by the conventional catalysts by increasing the temperature sustainability while providing high reactivity. However, the active catalytic metals as disclosed by Abe et al., still suffers from the problems that the active catalytic layer is vulnerable to the attack of the catalytic poisons. The sulfur oxides, phosphorous and zinc compounds contained in the exhaust gas may all cause a performance degradation of the the active catalytic layer in Abe's catalyst.
In yet another U.S. Pat. No. 5,182,249 entitled `Non-precious Metal Three-way Catalyst`, (Issued on Jan. 26, 1993), Wang et al. disclose a three way catalyst made of mixtures of catalytic components consisting of rare-earth metals and non-precious metal oxides. The catalyst includes a ceramic material as a first carrier comprising a mixture of metallic oxides stabilized by oxides of lanthanide elements disposed on or impregnated into the first carrier. A catalytically active layer structure including a second carrier and an oxides of rare earth metals are coated over the first carrier layer. The catalyst as disclosed by Wang et al. may be employed to reduce the manufacture cost because no precious metal is used. However, the practical usefulness of this catalyst converter is probability quite limited due to the fact that the conversion efficiency is likely not sufficient to be installed on an automobile exhaust pipe or an other kinds of internal combustion engines for actual applications.
Therefore, there is still a demand in the art of exhaust-gas catalyst manufacture to provide a catalytic composition and structure to overcome the technical difficulties encountered in the prior art. Specifically, this catalyst must be able to provide high reactivity rate while maintaining high resistance to catalytic poisons and capable of continuous and reliable operation with high degree of stability at high temperature and harsh vibrational environment for long period of time.